The focus of the proposed work is a 2-oscillator model (Pittendrigh & Daan, 1976) of the "complex" pacemaking system that drives circadian rhythms in mammals and is causally involved in the photoperiodic control of their reproductive activity. The experiments fall into three general groups. The first exploits a recent technical breakthrough in our laboratory that opens up the model to rigorous testing and development: we can now measure the phase-response-curves (PRCs) of mammalian circadian pacemakers with previously unmatched precision and at a rate (2 PRCs every month) that makes it possible for the first time to track the detailed time-course of the pacemaker which the model predicts is strongly history-dependent. Our experiments examine the history dependence of pacemaker time-course (PRC shape) and correlate this with history dependent changes in free-running period (tau) and activity time (alpha) observed directly from the activity rhythm. These experiments go hand in hand with further development of the model by computer simulation techniques. The second group focuses on the role of the circadian pacemaker in the photoperiodic control of reproduction in hamsters. Here the experiments ask whether pacemaker complexity, as evidenced by history dependent changes in alpha, tau, and PRC shape and by changes in the time course of pineal melatonin synthesis, play a role in hamster photoperiodism. The third group of experiments concern the function of the pineal gland and the suprachiasmatic nuclei (SCN). One subgroup of experiments test the hypothesis that the pineal gland and its hormone melatonin play a role in mediating the mutual coupling of the oscillations comprising the complex pacemaker. The final group asks whether the constituent oscillators can be localized within the SCN and explores the extent to which the history dependence of the pacemaker state depends on an intact SCN.